To carry a saw chain around the nose of a chain saw guide bar, a thin sprocket with teeth has been used for a long time. These carry the saw chain without friction against the edge of the guide bar nose. This causes less wear and needs less power. It also makes it possible in certain situations to cut with the upper, outward moving part of the chain.
To make a thin sprocket work with teeth so that it functions properly, it is necessary that the saw chain is carried at a well defined radius from the center of the sprocket bearing. This is accomplished if the ends of the chain drive link at its end portions are carried by the flanks of the sprocket teeth. When sawing timber with large chain velocity and coarse sawdust, it is important that there is space for sawdust between the underside of the drive links and the bottom of the gullet between the sprocket teeth, and that the drive link does not hit the sprocket in such a way that the pointed bottom part of the drive link is deformed, since its shape is vital for the distribution of lubricating oil along the path of the saw chain along the edge of the saw guide bar.
The gullet bottom has been shaped in different ways. In the patent U.S. Pat. No. 3,124,177 the gullet is V-shaped with straight flanks, with the disadvantage of concentrated stresses at the gullet bottom due to wedging action when the drive link is pushed into the gullet. In the patent U.S. Pat. No. 3,279,508 the gullet bottom is shaped as a circular arc, tangent to straight portions of the flanks, but this does not leave ample space for sawdust under the drive link. In the patents U.S. Pat. Nos. 3,263,715, 3,498,346 and 3,589,415 the gullet bottom was made as a circular arc, undercutting the straight flank portions so that the teeth were somewhat undercut. The latter shape leaves enough room for sawdust and has been much used.
Chain saws for mineral or coal use low chain velocity and small sawdust, and have less need for free space under the drive links.
There is a demand for nose sprockets with smaller diameters and fewer teeth, but it has until now been difficult to combine sufficient space for sawdust with sufficient material thickness between the gullet bottom and the sprocket bearing, as spacious, undercut gullets have a larger depth. The same problem has long been known from drive sprockets of chain saws, and was to resolved by letting the sawdust leave the gullets in an axial direction but this is not possible in a nose sprocket.